CONTENTS Magnox Graphite Core Decommissioning and Disposal
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CONTENTS Magnox Graphite Core Decommissioning and Disposal Issues...............................................1 M.E. Pick Graphite Waste Treatment and Disposal – A UK Perspective on the Current Opportunities and Issues .....................................................................................15 J. McKinney, S. Barlow Current Status and Future Objectives for Graphite and Radium-bearing Waste Disposal Studies in France .....................................................................................23 O. Ozanam Aspects of Graphite Disposal and the Relationship to Risk: A Socio-Technical Problem ..............................................................................................27 G.B. Neighbour, M.A. McGuire Radiation Damage in Graphite — a New Model....................................................................39 M.I. Heggie, I. Suarez-Martinez, G. Savini, G.L. Haffenden, J.M. Campanera Thermodynamic Modelling of an Irradiated Reactor Graphite Thermochemical Treatment Process .............................................................................................................47 S.A. Dmitriev, O.K. Karlina, V.L. Klimov, G.Yu. Pavlova, M.I. Ojovan Current Status of the Radiological Characterization of the Irradiated Graphite from RBMK-1500 Reactor in Lithuania ...........................................................................57 V. Remeikis, D. Ancius, A. Plukis, R. Plukiene, D. Ridikas, A. Smaizys, E. Narkunas, P. Poskas Decontamination of Nuclear Graphite by Thermal Methods .................................................77 J. Fachinger GLEEP Graphite Core Removal and Disposal .......................................................................83 M. Grave Review of the Characterization of Nuclear Graphites in UK Reactors Scheduled for Decommissioning.......................................................................................99 A. Jones, L. McDermott, B. Marsden and T.J. Marrow Graphite Dust Explosibility in Decommissioning: A Demonstration of Minimal Risk.......109 A. Wickham and L. Rahmani Magnox graphite core decommissioning and disposal issues M.E. Pick Magnox South, Gloucester, United Kingdom Abstract. Graphite core dismantling and disposal will be a key issue for the decommissioning of the United Kingdom (UK) Magnox reactors. The irradiated graphite arisings from the UK gas cooled reactor programme represent a significant proportion of the radioactive wastes currently destined for the UK geological repository. Data on the graphite and radionuclide inventory of the Magnox reactor graphite cores are presented together with data on the core designs. Magnox reactor cores represent a significant fraction of the worldwide irradiated graphite inventory and the paper recognizes that there may be alternatives to geological disposal. Sources and arisings of carbon-14, which is one of the major long lived radionuclides of concern, are discussed along with wider aspects of the arisings and behaviour of carbon-14 in the environment. Indubitably, core graphite disposal and the technical challenges it poses is one of the major issues to address in achieving final site clearance in a cost effective manner and reducing the liability cost associated with disposal of graphite. 1. Introduction The major large volume irradiated materials at final dismantling of Magnox reactors are: - Graphite - Steels - Concrete Each has particular challenges; irradiated graphite represents one of the largest volumes of irradiated materials and poses particular technical challenges. It is also a material for which there are a range of possible treatment and disposal options. The United Kingdom (UK) NDA (Nuclear Decommissioning Authority) Business Plan, 2008–2011, (Ref.[1]) states that “We believe that, due to the absence of a solution for the disposition of activated graphite, it is not yet possible to make a business case for accelerating Magnox decommissioning. Nevertheless, we will complete work on the business case in line with our Strategy for discussion with Government and, subject to availability of funding and viable waste disposal routes, will continue to explore the option of identifying a lead Magnox site to act as ‘test site’ for reactor decommissioning”. This statement reflects the fact that the NDA has expressed the aspirational goal to achieve site clearance in 25 years, i.e. one generation [2]. This will pose additional challenges through the reduction in the period of radioactive decay before dismantling takes place, which will result in wastes of higher unit activity being handled. The current strategy for the core graphite at final site clearance is to remove the graphite blocks and place in baskets in 4 m stainless steel containers with the option of encapsulation in cement. This will of course result in an increase in the overall volume of waste. Current Lifecycle Baseline (LCBL) plans envisage that final site clearance involving dismantling of the core will not be until approximately 85 years after reactor shutdown. 1 2. Inventory In total there are 26 Magnox reactors in the UK situated on 11 sites (Fig. 1). All of these are now shut down with the exception of Oldbury (due to close 2008) and Wylfa (due to close 2012). The size and mass of the graphite cores underwent a progressive increase from ~1100 t on the earliest reactor cores at Calder Hall and Chapelcross to 5500 t on Wylfa reactor cores. Arisings of core graphite from individual Magnox reactor cores are shown in Fig. 2. • Magnox reactors - 26 units on 11 sites Operational Permanently • 22 units have been Shutdown permanently shutdown Hunterston A Chapelcross • Remaining 4 will shut Calder Hall 2008 to 2010 Wylfa Traws fynydd Sizewell A Berkeley Bradwell Oldbury Hinkley Point A Dungeness A FIG. 1. Magnox reactor sites in the UK. Graphite Tonnes Per Reactor 6000 5000 4000 3000 2000 1000 0 Hall Wylfa fynydd ness A Bradwell Berkeley Oldbury Sizewell A Calder Traws kley Point A Chapelcross HunterstonDunge A Hin FIG. 2. Graphite core inventory of UK Magnox reactors. The total arisings of Magnox reactor core graphite in the UK are of the order 45 600 m3, which equates to around 57 000 t using a bulk density of 1.25 t m–3. The core graphite arisings are given in the UK Radioactive Waste Inventory [3] which quotes the radionuclide inventory at 100 years after shutdown as this corresponds to the assumed dismantling date. In addition, there are about 2300 t of graphite fuel struts and sleeves which were employed on the Berkeley and Hunterston A reactors; these arisings are stored in vaults on these sites. The core graphite is a mixture of Intermediate Level Waste (ILW) and Low Level Waste (LLW) depending on the core region from which it arose and the neutron flux it has been exposed to. The Magnox core graphite may be divided into the following components: 2 Moderator - Typically ILW Side Reflector - LLW/ILW Bottom Reflector - Typically LLW Top Reflector - LLW/ILW The Magnox core graphite was termed Pile Grade and was manufactured from petroleum coke by a synthetic route. Pile Grade A (PGA) graphite with a density of 1.70 g cm–3 was used for the moderator cores while Pile Grade B (PGB) graphite with a density of 1.64 g cm–3 was used for the reflectors. The graphite produced for the AGRs (Advanced Gas Cooled Reactors) in the UK was a higher density gilsocarbon with a density of 1.85 g cm–3 and used naturally occurring graphite as the filler source material and petroleum coke as the binder. 3. Radioactive inventory In the context of disposal, the major radionuclides of concern due to their long half-life are carbon-14 and chlorine-36. At shutdown of the reactors tritium is the predominant radionuclide in terms of Bq of radioactivity, but as this decays with a 12.3 year half-life after 30 years post-shutdown, carbon-14, which has a half-life of 5730 years, becomes the predominant radionuclide in terms of radioactivity. With respect to radiation dose cobalt-60, which has a half-life of 5.27 years, is the dominant contributor at shutdown and remains a major contributor for about 60 years after which the dose rates stabilize at a level about four orders of magnitude less than those at shutdown. Typical dose rates from the bulk graphite are 10 mSv h–1 at shutdown, falling to 1 mSv h–1 after 20 years, 0.1 mSv h–1 after 40 years and 0.001 mSv–1 after 80 years. The reduction in dose rate is mainly a consequence of cobalt-60 decay. The UK Radioactive Waste Inventory [3] shows an overall average total of carbon-14 in the Magnox core graphite of about 3200 TBq. Carbon-14 Carbon-14 is produced in gas-cooled reactors by neutron activation. The relevant reactions are: (a) 13C (n,γ) 14C (b) 14N (n,p) 14C (c) 17O (n,α) 14C Carbon-13 is a naturally occurring isotope (1.11%) of carbon. Nitrogen-14 is the predominant isotope of nitrogen (99.63%) and occurs as an impurity in graphite at ppm levels; however, the neutron activation cross-section is relatively large so this route is a significant source of carbon-14 in graphite. Carbon-14 is also produced in the carbon dioxide coolant and an additional source here is activation of the naturally occurring oxygen-17 isotope which constitutes 0.037% of naturally occurring oxygen. A major issue in assessing the carbon-14 inventory of graphite is the level of nitrogen in the graphite. The first reported specification for graphite for the UK nuclear industry is in a report by Rose and Shaw [4] dating from 1956. This report listed desired physical and mechanical properties along with chemical composition.